The utilization of conjugated dienes (eg. butadiene, isoprene) in polymerization or co-polymerization reactions for preparing synthetic rubbers has been widely used in industry for commercial production. Basically, these polymers can be prepared by using either emulsion (free radical polymerization) or solution (anionic polymerization) processes. Both processes give conjugated diene polymers (copolymers) containing unsaturated double bonds in the polymer backbone. These unsaturated double bonds can be further utilized for vulcanization to improve the toughness of the material. However, the fact that these unsaturated double bonds are vulnerable toward oxidation caused disadvantages of the material in their lack of stability at elevated temperature or under weathering (exposure to ozone). Particularly, styrene-conjugated diene block copolymers (eg. SBS, SIS), which are used in their unvulcanized state as thermoplastic elastomers, impact-resistanit modifiers and compounding additives, have been recognized for their need to improve their deficiency in thermal and weathering stability.
This deficiency in thermal and weathering stability can be improved by removing or reducing the unsaturated double bonds in the polymer chain. Thereby, a more stable material can be prepared by hydrogenation of the olefinic unsaturated double bonds to give polymers with nearly an aliphatic structure. Technically, the reduction of unsaturated double bonds can be achieved by using either heterogeneous or homogeneous catalysts. In general, heterogeneous catalyst shows much lower hydrogenation reactivity, thereby a higher reaction temperature, a higher reaction pressure and a larger quantity of heterogeneous catalyst are used in the process. Thereby, the process is economically not favorable. In addition to the above problem, the severe reaction conditions also results in the hydrogenation of not only the targeted olefinic double bonds but also hydrogenation of the aromatic double bonds. Consequently, the reaction leads to the production of an undesired polymer structure which contains undesired partial cyclohexyl structure (derived from hydrogenation of the benzene ring in the polymer backbone) and has undesired semi-crystalline properties. Therefore, it has been strongly desired in industry for development of a homogeneouls catalyst system that can be used for hydrogenation of conjugated diene polymers with high activity and high selectivity (without hydrogenation of the aromatic double bonds).
An effective homogeneous hydrogenation reaction is known using bis (cyclopentadienyl) titanium compounds as the reaction catalyst, (as disclosed by M. F. Sloan et al. in the Journal of American Chemical Society 1965, 85, 4014.about.4018, by Y. Tajima, et al. in the Journal of Organic Chemistry, 1968, 33, 1689.about.1690, in British patent 2,134,909 and in Japanese patent 61,28507). The catalyst system shows good activity and excellent selectivity for hydrogenation of the olefinic double bonds. however, because of the lack of stability of the catalyst system, the reaction can be hardly reproducible.
In 1985, Kishimoto et al. disclosed (in U.S. Pat. No. 4,501,857) the hydrogenation of the olefinic double bonds in the presence of at least one bis(cyclopentadienyl) titanium compound and at least one hydrocarbon lithium compound wherein the hydrocarbon lithium compound can be used in combination with the anionic living chain end. In 1987, Kishimoto et al. also disclosed (in U.S. Pat. No. 4,673,714) the usage of bis(cyclopentadienyl) titanium diaryl compounds for hydrogenation of conjugated diene based polymers that do not require the addition of alkyl lithium compounds. Both processes described above involve using a high concentration of bis(cyclopentadienyl) titanium compounds as a catalyst, and hence is not economically favorable. In 1990, Teramoto et al. revealed (in U.S. Pat. No. 4,980,421) that a similar hydrogenation activity can be achieved by using the same titanium compound in combination with an alkoxy compound. In 1991, Chamberlain et al. revealed (in U.S. Pat. No. 5,039,755) a process of using hydrogen gas to terminate the living chain followed by adding bis(cyclopentadienyl) titanium compounds to cause the hydrogenation reaction to proceed. The problem of the process described above is that hydrogen gas is not very effective for the termination of the living chain end, thereby the hydrogenation reaction is hardly reproducible. In 1992, Chamberlain et al revealed (both in U.S. Pat. Nos. 5,132,372 and 5,173,537) similar hydrogenation processes by using hydrogen gas to terminate the living chain end followed by adding bis(cyclopentadienyl) titanium compounds as well as an additional catalyst promoter (methyl benzoate) for effective hydrogenation of conjugated diene polymers.
All of the catalyst compositions described above involve using alkyl lithium (lithium hydride generated statics in situ) or alkoxy lithium compounds to activate the bis(cyclopentadienyl) titanium catalyst for effective hydrogenation of the conjugated diene polymers. It should be noted that in the presence of lithium species can also induce the reduction of the bis(cyclopentadicnyl) titanium compounds from Ti (IV) to Ti (III), resulting in the decomposition of the catalyst component as well as reduction of the catalyst activity and stability. Therefore, from economic considerations, it is highly desirable to develop a catalyst composition which can prevent the decomposition of the reactive catalyst species, bis(cyclopentadienyl) titanium compounds, and providing stable and effective hydrogenation results by using minimum amount of the catalyst species.